The present invention relates generally to digital versatile discs and, more particularly, to a method for making a substrate for a digital versatile disc, a digital versatile disc including the substrate, and a method for making a stamper for use in the manufacture of a substrate for a digital versatile disc.
The latest generation of optical disc technology is the digital versatile disc (DVD), which is sometimes referred to as a digital video disc. A DVD has significantly more data capacity than a compact disc (CD). For example, the data capacity of DVD 5 is about 4.7 GB, which corresponds to the data capacity of about seven CDs. The data capacity of DVD 9 is about 8.5 GB, which corresponds to the data capacity of about thirteen CDs.
In the manufacturing of current DVDs, two 0.6 mm polycarbonate substrates are bonded together with an adhesive material. These substrates are typically formed by an injection molding process in which a stamper is disposed within a mold cavity. The stamper, which is typically comprised of nickel, has a pitted surface on the side facing the mold cavity. This pitted surface forms a pitted surface corresponding to recorded data in the side of the substrate facing the stamper. The stamper is typically held in the mold cavity using a vacuum technique or a mechanical technique, e.g., a bushing. A central hole is provided in the stamper to facilitate alignment of the stamper within the mold cavity.
The central hole in stampers used in conventional CD and DVD manufacturing processes has been heretofore punched from the pitted side of the stamper. The central hole is defined by a surface of the stamper having two edges: one edge on the pitted side and the other edge on the back side. When the central hole is punched from the pitted side, the edge on the pitted side has a small radius, i.e., a chamfer or bevel, and the edge on the back side has a small burr. FIG. 1 shows stamper 10 having a central hole punched from the pitted side disposed within a mold cavity using a vacuum hold technique. As shown therein, burr 12 on the back side edge prevents stamper 10 from resting squarely against mold 14. Consequently, small gap 16 may be formed between the back side of stamper 10 and mold 14. A small gap 18 also may be formed between the surface of stamper 10 that defines the central hole and mold protrusion 20 that defines a groove in the substrate.
Punching the central hole in the stamper from the pitted side is undesirable for at least two reasons. First, the gaps between the stamper and the mold cause the stamper to move or bend when polycarbonate is injected into the mold cavity. This results in increased wear and poor mold quality. Second, the molded substrate has a burr that sticks out from the pitted side thereof. This burr causes centering and alignment problems when two substrates are bonded together with a thin layer of adhesive material to form a DVD.
FIG. 2 illustrates how a burr on the pitted side of the substrate interferes with the bonding of two substrates to form a DVD. As shown therein, opposing burrs 24a and 24b prevent substrates 22a and 22b from resting squarely against one another. Consequently, centering and alignment of the substrates is problematic. The distance, x, by which substrates 22a and 22b are out of alignment depends on the sizes of burrs 24a and 24b. Moreover, even if the substrates are centered properly, once released they may spring out of alignment.
In view of the foregoing, there is a need for a DVD substrate that does not have a burr that sticks out from the pitted side thereof.
Broadly speaking, the present invention fills this need by providing a stamper having a central hole that is punched from the back side thereof. When such a stamper is used to form a substrate for a DVD, the substrate does not have a burr that sticks out from the pitted side thereof.
In one aspect of the present invention, a method for making a substrate for a DVD is provided. In this method a stamper is first provided. The stamper has a front side with a pitted surface, a back side, and a central opening. The central opening is defined by a surface having an edge on the front side and an edge on the back side. A burr is formed on the edge of the surface on the front side of the stamper. The stamper is disposed within a mold such that the front side faces the mold cavity. Thereafter, the substrate is molded in the mold. In the molding process, the burr on the front side of the stamper forms a rounded edge in one of the walls defining an annular groove in the substrate.
In another aspect of the invention, a method for making a stamper for use in the manufacture of a substrate for a DVD is provided. In this method a stamper without a central hole is first provided. The stamper has a front side with a pitted surface and a back side. A central hole is then formed in the stamper from the back side thereof, preferably by punching. In one embodiment, the punch is aligned with an optical centering device arranged on a die side of the punch. In another embodiment, a pilot hole having a first diameter is first formed in the stamper. The central hole having a second diameter, which is greater than the first diameter, is then punched in the stamper from the back side thereof using the pilot hole to align the punch. In one embodiment, the punch is aligned with a guiding pin attached to the punch.
In yet another aspect of the invention, a DVD is provided. The DVD includes first and second substrates each having a central opening therein. The first substrate is disc-shaped and has a pitted surface with an annular groove formed therein. The second substrate is disc-shaped and has a surface with an annular groove formed therein. The grooves in the first and second substrates are defined by an inner wall and an outer wall having a rounded upper edge. The first and second substrates are bonded together such that the grooves in the pitted surface of the first substrate and the surface of the second substrate face one another and define an adhesive restraining cavity. In the case of a DVD 9, the surface of the second substrate also is pitted.
The present invention provides, among other things, a stamper having a central hole punched or otherwise formed from the back side thereof. Using this stamper, a substrate for making a DVD can be molded without having a burr form on the pitted side of the substrate. Thus, the present invention advantageously enables substrates to be bonded together to form a DVD using an extremely thin layer of adhesive material without the centering and alignment problems caused by burrs.
It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.